Sometimes people make insincere arguments to justify an action (or inaction), based upon the impact to a particular group.

For example, a person might say "Building that sports arena near my house is irresponsible! Think of how many homeless people we could house with that money!" In truth, the person does not want the arena near his or her house, and does not really care about whether homeless people are housed or not. What is the term used to describe the homeless people in this situation?

Another example might be large industrial agriculture organizations defending government policies "to protect the family farm" when these organizations are not family farms and do not care about protecting family farms. What is the term used to describe family farms here?

The best term I can think of is "smokescreen", as in "Homeless people are being used as a smokescreen to hide other concerns about the sports arena" but this seems informal, and there are probably better phrases (formal or informal). What are they?

Whoops, I just added red herring to my answer as you posted yours. I'll delete that part of my post now. Note, however, as I said in my answer, a red herring is usually applied to a false clue that leads to a desired conclusion, rather than the entire false justification proper.
– Dan BronOct 28 '14 at 12:16

In the example you give, the person justifying his/her position of opposing the construction of that arena in your example is what is referred to a nimby (an acronym for Not In My Back Yard). Quite literally in this case.
If you were to say "That's a typical nimby smokescreen", I believe everyone will get your meaning.

If you're looking for a more general and official name for the type of argument or fallacy given here, I think "Argumentum ad Populum" (translates roughly to "argument directed at the people") fits best. These types of arguments are generally used in heated debates in order to stir the masses. They try to appeal to the emotions. Crying outrage and oversimplification of the facts is part of this, just as pointing out social injustices is.
An example of a populist politician in a similar situation could be:

The arena in and of itself is not a problem. It creates job opportunities for the people, but the contract was sold to a foreign company. This whole project is a farce, yet another example of how a corrupt government is unwilling to do something about the high unemployment rates.

This is not a literal quote, but we've all heard rants that boiled down to this...

Because the person is arguing that the money being spent could've gone to helping the homeless, one could argue that there's also a logical fallacy of correlation and causation. Not the full-on post hoc, ergo propter hoc (because of the arena, people are homeless), but more a suggestion along the lines of: Because of the arena, more people will remain homeless.

All in all, smokescreen, decoy, cover story, and all other suggestions are fine. Just thought I'd put in a couple of logical fallacies that can be used to describe these kinds of arguments.

Note that I referenced NIMBY in a footnote to my answer, but note also that while NIMBY beat characterizes the specific example OP provides, it's not applicable to the broader phenomenon of false pretexts (facades or whatever).
– Dan BronOct 28 '14 at 12:59

@DanBron: Didn't see your footnote, but you're right, I'll edit to make it clear that the nimby bit applies to this specific example more than it does to the fallacy in general
– Elias Van OotegemOct 28 '14 at 13:05

Sorry to keep bugging you, but wouldn't argumentum ad populum be more properly cast as "argument from popularity"? That is, evidencing your argument on the fact that a lot of people already believe it, rather than "an argument intended to rouse the people"?
– Dan BronOct 28 '14 at 13:18

@DanBron: There are several forms of the argumentum ad populum. The more common forms are arguments from popularity or generally held beliefs, but that's not always the case. War rhetoric is a nice example of this: ("You're either with us or against us") or playing the patriot card ("You're opposed to X, so you're unpatriotic") are both 'arguments' used to create a black/white version of reality, in an attempt to play the masses. Either way, "Argumentum ad populum" still translates to "Argument to the people", and not from the people/popularity
– Elias Van OotegemOct 28 '14 at 13:32

Ah, I didn't know the term was applied that way. Thank you!
– Dan BronOct 28 '14 at 13:34

It's called rationalization.
You know your argument is not reasonable, defensible, logical. However, you would want to make it aceptable, by give it a false rationale, an artificial justification. So you 'rationalize' it with a clever argument like in the instant examples.

A person who is blamed for the wrongdoings, mistakes, or faults of
others, especially for reasons of expediency.

To be honest there is a little difference, because in this case, people are blamed instead of others. It all depends on what do you want to say. If scapegoat doesn't fits your needs, I would use just smokescreen if I was you :)

But in OP's example, the homeless people aren't being blamed (accused of wrongdoing). Quite the opposite, in fact. FYI, despite my comments, I was not the one who downvoted you.
– Dan BronOct 28 '14 at 12:23

It is okay, I don't care who downvoted. I just thought the word could eventually apply for its case and thought it was right to give an help. Suggesting something isn't priced. Unfortunately, in this community, you can randomly downvote, and that's what you get for trying to give an advice.
– Marcello SilvestriOct 28 '14 at 16:17